Flux powder for use during continuous casting of steel and method of producing same

ABSTRACT

A FLUX POWDER AND METHOD OF PRODUCING SAME FOR USE DURING CONTINUOUS CASTING OF STEEL, CONTAINING PORTLAND CEMENT, A FLUXING AGENT, AND A CARBON CARRIER, WHEREIN THE CARBON CARRIER POSSESS A GRAIN SIZE LESS THAN 1$, THE ALUMINUM OXIDE CONTENT OF THE POWDER IS IN THE RANGE OF 2-12% BY WEIGHT, AND THE LIME-SILICA RATIO IS ADJUSTED TO BE IN THE RANGE OF 0.7-1.0 THROUGH THE ADDITION OF QUARTZ POWDER.

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US. Cl. 75-94 9 ClaIml ABSTRACT OF THE DISCLOSURE A fiux wder and method of producing same for use dunn fconiinuous casting of steel, containing poriland cement, a fluxingggent, and a carbon carrier, wherein the carbon carrifiissess a grain sfi less than la, the aluminum oxide content of the powder is in the range of 2-12?! by weight, and the lime-silica ratio is adjusted to be m the range of 0.7-1.0 through the addition of quartz powder.

BACKGROUND OF THE INVENTION The present invention relates to an improved flux or slag powder for use in the continuous casting of steel, and of the type containing portland cement, a fluxing agent and a carbon carrier, and the invention also pertains to a method of producing such fluxor slag powder.

During continuous casting, the metal bath in the continuous casting mold is oftentimes covered with a flux or slag powder. It absorbs non-metallic contaminants or foreign particles, functions as a lubricant between the wall of the mold and the continuously cast strand or casting, prevents oxidation of the steel and reduces thermal radiation.

Prior art flux powders almost exclusively employ as the basic material fiy ash or flue dust, a by-product of thermal power plants. The use of this material is quite problematic since its chemical and structural composition fluctuates markedly, and therefor, the thermal-physical properties are appreciably different. Such constitutes a decisive drawback particularly for continuous casting applications. In order to obtain good surface properties of a semi-finished product produced from strands of blocks and to attain satisfactory lubrication, there is furthermore required uniformly melting powder. Moreover, health reasons re quire the slag powder to be free of fluorite to prevent the formation of noxious vapors.

There is known to the art a slag or flux powder, the composition of which contains, among other things, portland cement, sodium carbonate, and carbon donors. Due to the likewise present refractory clay powder, the con tent of alumina is undesirably raised, so that with absorption of alumina from the steel bath, the dissolving capacity of the slag for further absorption of alumina becomes increasingly more difi'icult, which is disadvantageous during casting of aluminum killed steels. Furthermore, there is used as the carbon donor coke fines or graphite powder, the average grain size of which is relatively coarse and the grain size distribution variable, so that it is practically impossible to carry out an exact adjustment of the melting behavior.

- SUMMARY OF THE INVENTION Hence, from what has been explained above, it should be apparent that this particular field of technology is still in need of a flux or slag powder for use during the continuous casting of steel which is not associated with the aforementioned drawbacks and limitations of the prior art proposals. It is therefore a primary object of the present invention to provide an improved flux powder for continuous casting of steel which is not associated with the previously discussed drawbacks and effectively and reliably fulfills the existing need in the art.

Another and more specific object of the present invention aims at the provision of a flux powder, the melting rate of which can be adjusted and which in the melted state is capable, when required, of absorbing extremely large amounts of alumina with only a slight increase in its melting temperature, and furthermore is capable of uniformly flowing-off through the gap between the mold and the continuously cast strand, so that tapping of the slag is no longer necessary.

A further object of the present invention relates to an improved flux powder, and method for the production thereof, which possesses an extremely precise consistency in its composition or analysis and tree of halogen compounds, such as fluorite.

Of course, the powder of this development should also be used for casting steels which do not precipitate or separate out any alumina (clay) or aluminum oxide respectively, for instance certain rustproof or corrosion resistant steels predominantly treated with silicon. Through the use of the powder there should be obtained cast products which are free from surface defects, such as lengthwise or transverse fissures or cracks or markedly defined oscillation marks.

Furthermore, the powder during use, especially when added to the melt, should develop as little dust a possible and likewise as little smoke as possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Considering the invention in greater detail, the flux or slag powder of this development which is contemplated for use in the continuous casting of steel contains portland cement, a fluxing agent and a carbon carrier. The carbon carrier has a grain size of less than 1,, the aluminum oxide content is in the range between 2 and 12% by weight, and the lime-silica (silicic acid) ratio is adjusted to be in the range between 0.7 and 1.0 through the addition of quartz powder.

The basic material of the flux powder consists of S0 to by weight portland cement. Furthermore, as is well known in this particular art, the powder contains as the fluxing agent an anhydrous alkali carrier, for instance calcinated sodium carbonate which is present in an amount of 20% by weight.

It is an important aspect of the invention that the flux powder contain a real fine carbon carrier, that is to say, a carbon carrier with a grain or granulation size below lg, since apart from the absolute content of carbon carrier its fineness is decisive and thereby it is possible to adjust the melting or fusing rate of the powder. By virtue of the aforementioned fineness or granulation size of the carbon carrier, there is increased both the temperature of the slag and also the insulation effect of the not yet molten or fused powder. It has been found that there is a decisive beneficial correlation between the granulation size of the carbon carrier and the necessary content thereof for the powder, and specifically the finer the grain size of the carbon carrier, the less content of carbon carrier is required in the flux powder in order to attain the desired effects. This affords the further advantage during casting of certain steels for instance rustproof materials of low carbon content, with the flux powder, that there can be extensively prevented undesired carbonization. There is advantageously employed as the carbon carrier carbon black in an amount of l8% by weight. This material constitutes a readily accessible starting material and possesses constant properties and offers guarantees for faultless, uniform melting. Furthermore, it is most economical in the required fineness or granulation size as the carbon carrier. By varying the quantitative amounts of carbon black which are added within the indicated range, it is possible to adjust to a desired degree the melting rate. Already with a relatively small increase in the quantity of carbon black, the heat withdrawn per unit of time from the steel is thus absorbed by a thinner slag layer, so that such becomes hotter and there is less tendency for forming damaging slag threads or fibers along the periphery of the molten metal bath. With small casting velocities and larger size strand dimensions or formats and low casting temperatures, there is added more carbon, with greater casting velocities and smaller strand dimensions or formats, which affords a larger supply of heat per unit of surface of the molten bath in the mold, there is added less carbon.

The A1 0 content in the powder, advantageously should amount to between 212% by weight for aluminum-free steels, for steels containing aluminum between 27% by weight, which can be insured for by the content of alumina or clay in the basic material portland cement with or without the admixture of alumina or clay. The low content of A1 0 in the range of 27% by weight for casting aluminum-containing steels affords two advantages: firstly, the dissolving capacity of the molten slag is increased for precipitation of the alumina or clay of the liquid steel with as little as possible increase in the melting temperature of the slag. As a result, there is realized with the inventive conditions, that is to say, especially with a lime-silica ratio which is adjusted to be between 0.7 and 1.0, a flowing-off of the slag into the shrinkage gap between the cast strand and the mold, and the desired lubricating conditions are maintained. Secondly, the melting temperature of the slag is initially lowered owing to the reaction of the alumina which at the start is absorbed from the steel with the oxidic constituents already present in the slag. Lowering of the melting point facilitates the flow behavior of the slag and renders more difficult a continual enrichment of the alumina in the slag blanket. There does not occur any impermissible thickening thereof and tapping of the slag is not required.

Furthermore, in contrast to the prior art powders, the inventive powder possesses the notable beneficial advantage that there does not occur any development of dust and smoke, suppressing health dangers which otherwise would be present. Furthermore, there could be ascertained an improved trickling effect, which is favorable in terms of an automatic compensation of the pour height of the powder layer in the mold.

The lime-silica ratio of the powder is adjusted to a value between 0.7 and 1.0 through the addition of up to 30% by weight quartz powder, of a purity of about 99.5%. Quartz, with its high melting point, has, as a component of the flux or slag powder, the advantage that when the powder is used it first leads to the formation of a lowmelting slag phase when the insulating carbon in the powder is decomposed.

Owing to the purity and uniformity of the starting substances employed for the fabrication of the flux powder, the latter always possesses constant mineralogical and analysis properties and therefore constant behavior in the mold with regard to melting, insulating, solubility and lubricating properties.

In order to further explain the invention, there will now be given an example of the use of the inventive powder during continuous casting of steel: aluminum killed steel is cast into a slab of the dimensions 2050 x 200 with a casting speed of 0.7 m./min. There is added to the molten metal bath in the mold a powder of the following composition, the percentages constituting percent by weight:

Percent by weight 1 1 In the form of carbon black having a particle size beneath Through the addition of the quartz powder, there is regulated a lime-silica ratio of 0.85. The aluminum oxide content in the applied powder amounted to 3.2% by weight. The consumption of powder amounted to 0.6 kg./ton of steel. The slag layer at the molten metal bath was very hot and the lubricating action was outstanding owing to uniform flow-out properties. Tapping of the slag was not necessary. Since the powder was free of the otherwise usual additive of fluorite, there was no danger of health hazards. The surface of the slab did not exhibit any defects.

It is possible to replace apart of the required carbon black by other finely ground carbon carriers, wherein however there must be taken into account that in this instance the entire content of carbon in the powder must be greater in order to realize the desired effects.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

Accordingly,

What is claimed is:

1. A flux powder for use in the continuous casting of steel, containing portland cement, a fluxing agent, and a carbon carrier, the improvement comprising the carbon carrier possessing a grain size of less than lg, the powder having an aluminum oxide content in the range of 2-12 percent by weight, and a lime-silica ratio adjusted between 0.7 and 1.0 through the addition of quartz powder.

2. The flux powder as defined in claim 1, wherein the carbon carrier is carbon black containing carbon in an amount of 1-8 percent by weight.

3. The flux powder as defined in claim 1, especially for casting aluminum killed steel, wherein the aluminum oxide content is in a range between 2-7 percent by weight.

4. The flux powder as defined in claim 1, wherein the fluxing agent is calcinated sodium carbonate which is present in an amount of 5-20 percent by weight.

5. The fiux powder as defined in claim 1, wherein the portland cement content is in a range between 50-70 per cent by weight.

6. A flux powder for use in the continuous casting of steel, containing portland cement, a fiuxing agent, and a carbon carrier, the improvement comprising the carbon carrier possessing a grain size of less than l t, the powder having an aluminum oxide content in the range of 2-12 percent by weight, and a lime-silica ratio between 0.7 and 1.0.

7. A method of producing a composition useful as a flux powder for continuous casting of steel and containing portland cement, a fluxing agent, and carbon carrier, the improvement comprising the steps of controlling" the granulation size of the carbon carrier so as to be beneath In, controlling the aluminum oxide content of the powder to be between 2 and 12 percent by weight, and adding quartz powder in an amount sufiicient to adjusting the lime-silica ratio to be between 0.7 and 1.0.

8. The method as defined in claim 7, further including the step of adding carbon black as the carbon carrier such that the quantity of carbon is in a range of 1-8 percent by weight.

9. The method as defined in claim 7, further including the step of regulating the aluminum oxide content to be in a range between 2-7 percent by weight.

References Cited UNITED STATES PATENTS 2,375,520 5/1945 Bowden et a1 75-94 X 2,620,267 12/1952 Kern 10676 X 2,343,760 3/ 1944 Fleming et al. 75-94 X 3,516,821 6/1970 Neu 7594 X 3,713,813 1/1973 Lundin 7594 X 1,364,004 12/1920 Stowell 10676 ALLEN B. CURTIS, Primary Examiner 16 T. A. WALTZ, Assistant Examiner 

